The speed reducer operating in the trochoid speed reduction system includes a ring-shaped internal gear attached to a housing, a disc-shaped external gear a pitch diameter of which is set to be smaller than the that of the internal gear and which is arranged inside the internal gear and an output member linked to the external gear and outputting rotation movement of the external gear to the outside. The external gear performs revolution movement (oscillatory movement) at a given number of revolutions (revolution speed) while internal teeth and external teeth are meshed with each other by actuation from an electric motor inside the internal gear attached to the housing. Along with the revolution movement (oscillatory movement), the external gear performs rotation movement at a number of rotations (rotation speed) reduced lower than the revolution speed by meshing with the internal gear. Then, the reduced rotation movement of the external gear is transmitted to the output member, and a reduced rotation output is outputted from the output member.
For example, a speed reducer disclosed in FIG. 5 of PTL 1 (JP-A-2014-81068) includes a bar-shaped input shaft in which a rotor is attached to a tip portion and which rotates by rotation input from an electric motor, a ring-shaped internal gear attached to a housing, a ring-shaped external gear meshed with the internal gear and rotating around the input shaft and plural output pins provided between the external gear and an approximately disc-shaped output member.
The external gear is arranged in a state that part of outer teeth is meshed with inner teeth of a first internal gear inside the internal gear. Then, when the motor is activated and the input shaft rotates, the external gear performs revolution movement (oscillatory movement) in the internal gear at an input speed inputted from the motor as well as performs rotation movement at a rotation speed reduced with respect to the revolution movement at a given rate. Then, the rotation movement is transmitted to the output member through the output pins and the output member rotates around the input shaft rotating with the external gear (see PTL 1: JP-A-2014-81068)
As an eccentric oscillation type gear device, a gear device provided with a cross roller bearing between an output member and a casing to thereby receive loads in a thrust direction and a radial direction is also proposed (see PTL 2: JP-A-2017-44319).